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This lesson covers key concepts in chemical analysis from the AQA GCSE Combined Science Trilogy (8464) specification. You need to understand the scientific meaning of purity, what formulations are, and how chromatography works to separate and identify substances.
In everyday language, "pure" means "not mixed with anything harmful." In chemistry, pure has a very specific meaning:
Key Definition: A pure substance is a single element or compound, not mixed with any other substance.
A pure substance has a sharp, fixed melting point and boiling point. An impure substance (mixture) melts and boils over a range of temperatures.
| Property | Pure Substance | Impure Substance (Mixture) |
|---|---|---|
| Melting point | Sharp and fixed | Melts over a range; lower than expected |
| Boiling point | Sharp and fixed | Boils over a range; higher than expected |
Example: Pure water boils at exactly 100°C and freezes at exactly 0°C. Adding salt to water raises the boiling point and lowers the freezing point.
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